Microscopic Polyangiitis

Definition AND EtiologyTop

Microscopic polyangiitis (MPA) is a necrotizing vasculitis with few or no immunologic deposits, which usually affects small vessels (arterioles, capillaries, venules) and may involve small- and middle-sized arteries. It very frequently coexists with necrotizing glomerulonephritis and with pulmonary capillaritis. There is no inflammation extending beyond the blood vessels or granulomatous inflammation, distinguishing it from granulomatosis with polyangiitis. MPA and the clinically similar and similarly treated granulomatosis with polyangiitis (GPA) are classified as antineutrophil cytoplasmic autoantibody (ANCA)–associated vasculitides (AAVs). The other conditions in this group are renal-limited vasculitis and eosinophilic granulomatosis with polyangiitis (EGPA).

Clinical Features and Natural HistoryTop

The course of MPA may be indolent, with recurring general symptoms of fever, weight loss, myalgia, and arthralgia persisting for many months or years prior to the onset of organ-specific symptoms. Organ specific manifestations include:

1) Cutaneous manifestations: Palpable purpura (observed at presentation in ~50% of patients). These tend to favor the feet, lower legs, and buttocks.

2) Renal manifestations: There can be mild renal involvement or rapidly progressive glomerulonephritis.

3) Nervous system manifestations: Mononeuritis multiplex (~60%).

4) Respiratory manifestations: Lung involvement can present as diffuse alveolar hemorrhage.

DiagnosisTop

Diagnosis is based on clinical manifestations and histologic examination of the skin, kidney, or lung biopsies. A positive myeloperoxidase (MPO) antineutrophil cytoplasmic antibody (ANCA) (type of perinuclear ANCA [p-ANCA], present in ~70% of patients) or proteinase 3 (PR3) ANCA (type of cytoplasmic ANCA [c-ANCA], present in 45%) test result also suggests the diagnosis of microscopic polyangiitis.

Diagnostic Tests

Laboratory test results may reveal elevations in the erythrocyte sedimentation rate and C-reactive protein (CRP) levels and features of glomerulonephritis. Chest radiographs, high-resolution computed tomography, and bronchoalveolar lavage reveal typical features of alveolar hemorrhage.

Differential Diagnosis

Polyarteritis nodosa (differences: only extraglomerular vessels of the kidneys are involved [no features of glomerulonephritis], pulmonary involvement usually does not occur, ANCA-negative); other causes of pulmonary-renal syndrome (see Granulomatosis With Polyangiitis), cutaneous leukocytoclastic angiitis.

TreatmentTop

As in granulomatosis with polyangiitis.